Conventional microscopes are based on two forms of radiation, electromagnetic waves as in the optical instruments and the electron waves as in electron microscopes.
Optical instruments have been refined over a period of many years to provide accurate images of even objects as small as biological cells. Regardless of such refinements, inherent limitations exist since the optical systems basically sense the dielectric properties of the specimen or object being imaged. Because of this, certain objects are optically transparent so that no image may be developed while others are optically opaque so that interior details are unrevealed. Furthermore, there are limitations in contrast sensitivity since, for example, there is little intrinsic optical contrast in certain biological specimens such as tissue sections and cell suspensions. Such contrast limitations have been but partially overcome by the very tedious technique of staining biological specimens.
The electron microscope of course is technically much more difficult to construct and use. Additionally, certain objects such as living cells cannot be examined because of the requirements for support in a vacuum and the electron bombardment which damage the cells.
The relatively recent development of acoustic wave generation at frequencies approximating 1,000 MHz provides an acoustic wavelength in water in the neighborhood of one micron and accordingly has suggested itself as a potentially excellent mechanism for the generation of high resolution images. Furthermore, it is the variation in the elastic rather than the dielectric properties of the specimen that determines the scattering, reflection, and absorption of the acoustic energy. This enables the study of details lying beneath the surface of certain specimens which would otherwise be unrevealed due to optical or electron opacity. Furthermore, and of the greatest importance, variations in the elastic properties also show different details, and, in particular, provide for large intrinsic acoustic contrast. As an example, in the Feb. 15, 1974, issue of APPLIED PHYSICS LETTERS an "Acoustic Microscope, Scanning Version" is described in an article appearing on pages 163ff as authored by Lemons and Quate, and as is pointed out has provided good resolution and sensitivity. Briefly, the nature of the object to be imaged is explored by the amount of perturbation or attenuation it offers the acoustic beam or by the reflections or phase shifts caused by changes in acoustical impedance across the object. Practical limitations in the resolution are of course imposed by the operating frequency and the resultant image, of course, is dependent upon the linear elastic properties of the object.